CA1307130C

CA1307130C - Pressure vessel sight glasses

Info

Publication number: CA1307130C
Application number: CA000527595A
Authority: CA
Inventors: Paul Hinz; Helmut Dislich; Gerhard Weber
Original assignee: Schott Glaswerke AG
Current assignee: Schott AG
Priority date: 1986-01-20
Filing date: 1987-01-19
Publication date: 1992-09-08
Anticipated expiration: 2009-09-08
Also published as: PT84139B; DE3601500A1; DE3770485D1; ATE64138T1; US4877668A; EP0231749A1; DE3601500C2; EP0231749B1; PT84139A; ES2023120B3

Abstract

Abstract of the Disclosure A pressure vessel sight glass, especially a tempered glass, is protected against the corrosive attack of normally alkaline, boiler feed water heated up to 300°C, by providing the sight glass with a metallic oxide coating consisting essentially of SnO2, optionally combined with Cr2O3. The coating is preferably applied as a sprayed solution of soluble salt convertible to the oxide.

Description

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PRESSURE VESSEL SIGHT GLhSSES

Background of the Invention This invention xelates to aorro~ion-resistant sight or inspection glasses ~or pressure vessels.
Alkaline aqueous solutions are frequently utilized in pressure vessels at t~mperatures of up to 300C, but the slgh~ glasses incorporated in a conventional manner in the side of such pressure vesseis generally do not exhibit adequate resistance to deterioration, irrespective of whether the glasses are hard or soft glasses, tempered or not. As a consequence, to avoid failure of the sight glass during operation o~ such a vessel, the sight glasses must be exchanged ~requently, which is expensive.
As an attempt to remedy the problem, mica plates have been placed in front o~ the sight glasses to protec~ the latter. Whereas, this result~ in an improvement since mica is more stable with respect to alkalis than the glasses employed, protection is limited and depends on the quality of the micas. Additionally, on account~ of the diminishing availability of optiGal grade micas, the cost thereo~ has risen dramatically in recent times and has even exceeded the price o~ the~sight glasses to be protected. Consèquently, the use of mica will become a prohibitively expensive solution to the problem of failure of sight glasses, particularly since the availability of optica] grade mica is expected to become worse in the future.
~nother attempted solution has ~een to coat the sight ~,, .

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glasses with a synthetic resin on at leas~ the interior side, i.e., the side of the glass in contack with the fluid inside the pres~ure vessel. One dxawback o~ this expedient is that the temperature stability of the plaskics employed is unsatisPactory above 200c whereas boilers ar~ frequently operated up to 300c. Even below 200c, damage to the synthetic resin and resultant separation from the glass substrate occur due to steam diffusion and erosion from the turbulent conditions at the boiler window. In all these cases, the entire sight glas~ must be replaced.
For thesë reasons, another suggested solution to the problem is based on providing the inspection glass with a transparent eynthetic re~in sheet resistant to aggres~ive, especially alkaline, solutions. This sheet is merely ~ixed in place mechanically and thus can be exchanged without replaclng the glass as well. For this purpose, transparent fluoropolymers have been sugges~ed, especially polytetra-fluoroethylenehexafluoropropylene copolymers according to Austrian Patent 3~9,790. These polymers are stable against typical, alkaline boiler feed waters containing.phosphate and carbonate ions at a pH of 10 and at up to 200C. But here again, there is the disadvantage that the stability is limited.to 200C. Another disadvantage is that steam diffuses through the sheet into the space between the sheet and the sight glass and hampers visibility. Also, the cost of sùch perfluoropolymers is considerable and, in conjunc-tion with the a~orementioned drawbacks, this solution to the problem, for all practical purposes,-is restricted to a few special cases. :
Summary of the Invention ~n object of this invention is to provide corrosion-resistant inspection or sight glasses withstanding higher operating temperatures, i.e., up to 300C.
Another object of the invention is to provide a process for the application o~ coatings to boiler sight gla~es so as to impart corrosion and erosion resistance to these sight glasses at temperatures up to 300C.

Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the ark.

To attain these objects, the sight glasses are coated at least on the interior side with stannic oxide (SnO2), and preferably in admixture with chromic oxide (Cr2O3). The coatings are preferably applied by spraying a solution of a precursor of the oxides, e.g., SnC14 and/or CrC13, on the glass at about 500-700C.

The sight glasses of this invention are particularly useful in applications where the boiler feedwater has a pH
above 7, and especially where the temperature in the boiler is above about 200C.

The sight glasses according to the present invention are thus characterized in that they are coated, at least on the interior side with one or more firmly adhsring metallic oxide layers exhibiting a substantially higher corrosion resistance than the substrate glasses proper, especially against alkaline aqueous solutions.

Although corrosion-preventive metallic oxide layers are known, they have not been disclosed as useful on sight glasses. The closest state o~ the art relates to coated E~
glass fibers resistant to alkaline cements, with the use of tin oxide and zirconium oxide as the metallic oxides showing a limited protection against alkaline attack. However, since protection has been directed to operations at ambient temperature, the behavior of the aforementioned metallic .f,`~

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oxides at higher temperatures up to 300C was neither known nor predictable; this holds true as well for other oxides and mixed oxides.

Although pertinent manuals and textbooks contain statements regarding the solubility of various oxides in forms such as powdexs, molded elements, or similar materials, these taachings cannot be applied analogously to coatings.
The reason for this is probably that not only is the chemical characteristic of the oxide important, but so is the structure of the layer, the bulk density of which can be high or low. In case of a low bulk density, the coating is more porous and thus more vulnerable and more permeable; at a high bulk density, the layer is less subject to attack and less permeable. The bulk density, in turn, depends on the type of coating method and the process parameters selected, and is also affected by the chemical and surface characteristics of the substrate to be coated. The same holds true for obtaining satisfactory adhesion of the oxide layer to the substrate.

A special feature of the present invention resides in the use of tempered inspection glasses. The tempering step is generally conducted after the coating step. Accordingly, the oxide layers must withstand the tempering schedule, i.e., temperatures of up to 800C, and sudden quenching resulting in a rapid change in expansion at various coefficients of expansion of the substrate and the layer. The oxide layers must likewise not lose their ability to function as corrosion preventives, i.e., no tearing or even crackling of the coating nor detachment from the substrate.

This invention is applicable to all conventional sight glass compositions.

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Although many corrosion-protection layers are known from the prior art, among these SnO2, they are effective only against mild stress, for example in accordance with DIN 50 018. In contrast, in the case of boiler feed waters up to 3000C, the attack of network linkages, e.g., si-o-si-, is so massive that, within days, several millimeters of thickness are normally eroded. Therefore, there was reason for a person skilled in the art of preparing sight glasse~ to believe that the known measures for protecting against mild attack would fail under the conditions found in boilers.
Clear proof of this fact is the use of the extremely expensive mica cover plates, an expedient almost exclusively employed in today's practice. Thus, besides mica and the fluoropolymers which have only limited usefulness, no other satisfactory protection was known heretofore for pressure vessel sight glasses. This confirms the observation although many corrosion protection coatings were known for other purposes, they were not believed to be applicable to the problems solved by the present invention.

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For the purposes of applying the coating to the sight glass, thic~ layers of generally about 0.3 to 1 micron are generally sprayed on in a single step.

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The resultant thick coatings are somewhat non-homogeneous optically, but this does not represent any serious disadvantage for their use as sight glasses.

A thus-produced SnO2 coating has an increased electrical conductivity and infrared reflection which, in case of usage as sight glasses~ is not only not required, but in some cases is even undesirable. These deleterious properties can be mitigated or entirely eliminated by using mixed oxides of SnO2 and Cr2O3. Infrared reflection is especially undesirable in the case of tempering of sight glasses, since the heating of the sight glasses to the required temperatures of up to 800C is hindered by heat reflection. For this reason, the incorporation of Cr203 is a preferred measure, as already described, in principle, in DAS 1,204,36g (published in 1965).

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

In the following examples, all temperatures are set forth uncorrected in degrees Celsius; unless otherwise indicated, all parts and percentages are by weight.

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~3~30 - 7a -Example 1 A coating of SnO2 having a thickness of about 0.8 um (micron) is provided from a solution of:

845 ml methyl alcohol 155 ml tin tetrachloride which is sprayed onto a boiler sight glass at temperatures of up to 700C. The coating is transparent for visible ~ -.

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light. Long-wave infrared light of the wavelengths > 3 um, required for tempering, is reflected to an extent o~ above 70~ and thus retards the re~uired heating-up process ~or tempering.

Example 2 3.8 g of chromium(III) chloride x 6 H20 is added to a solution according to Example 1 and i9 dissolved therein.
At 700C, a coating having a thickness of about 0.8 um is sprayed onto a boiler sight glass. The coating i8 trans-parent to vi~ible and long-wave IR light of > 3 um. The thus-coated ~nspqction glasses can be prestressed in a normal tempering process without the coating exerting any deleter~ous in1uence on the tempering operation.
ExamPle 3 Coatings produced according to Examples 1 and 2, consisting essentially of Sno2 and optionally Cr203 show excellent protective properties in alkaline boiler watersO
Thus, this coating prevents glass corrosion over at least 70 hours at 225C and 25 bar in an autoclave at a pH of 10. In co~parison, unprotected borosilicate glass, highly resistant per se, is dlssolved to the extent of 0.6 mm in the thick-nes~ during the same time period.
Brief Description of the Drawin~s In the Figure, the base glass 1 is shown to be grooved, with a metallic oxide layer, 2 superimposed thereon.

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~3~7~30 The preceding examples aan be repeated with similar succe~s by ~ubstltuting the generically or specifically described reactants and/or operating conditions o~ this invention for ~hose used in the preceding examples.
From the foregoing description, one skilled in thè art can easily ascertain the essential characteris~ics o~ this invention, and without departing from the spirit and scope ther~of, can make vaxious changes and modifications of the invention to adapt it to various usages and conditions.

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Claims

1. A sight glass suitable for pressure vessels containing boiler feed water at temperature up to about 300°C, said sight glass having a coating of stannic oxide at least on the surface of the sight glass arranged to face the interior of the vessel.

2. A sight glass according to claim 1, said stannic oxide coating additionally containing chromic oxide.

3. A sight glass according to claim 1, wherein the glass is a borosilicate glass.

4. A sight glass according to claim 1, wherein the coating is of a thickness of about 0.3 to about 1 micron.

5. A sight glass according to claim 3, wherein the coating is of a thickness of about 0.3 to about 1 micron.

6. In a pressure vessel fitted with a sight glass, the improvement wherein the sight glass has a coating of stannic oxide at least on the surface of the sight glass facing the interior of the pressure vessel.

7. A pressure vessel according to claim 6, said stannic oxide coating additionally containing chromic oxide.

8. A pressure vessel according to claim 6, wherein the sight glass is a borosilicate glass.

9. A pressure vessel according to claim 6, wherein the coating is of a thickness of about 0.3 to 1 micron.

10. A pressure vessel according to claim 8, wherein the coating is of a thickness of about 0.3 to 1 micron.

11. A sight glass according to claim 1, wherein the coating is applied by spraying a solution of a tin salt convertible to the oxide thereof, on the glass at about 500-700°C.

12. A sight glass according to claim 11, wherein the salt comprises SnC14.

13. In a process for heating boiler feed water having an alkaline pH to over 200°C up to 300°C in a pressure vessel fitted with a sight glass, the improvement comprising employing the pressure vessel of claim 6.

14. In a process for heating boiler feed water having an alkaline pH to over 200°C up to 300°C in a pressure vessel fitted with a sight glass, the improvement comprising employing the pressure vessel of claim 7.

15. In a process for heating boiler feed water having an alkaline pH to over 200°C up to 300°C in a pressure vessel fitted with a sight glass, the improvement comprising employing the pressure vessel of claim 9.

16. A process for the production of corrosion-resistant SnO2 layers on a borosilicate sight glass, comprising spraying a solution of a tin salt convertible to stannous oxide, onto the borosilicate glass at 500-700°C.

17. A process according to claim 16, wherein the solution comprises SnCl4.

18. A process according to claim 17, wherein the solution also contains CrCl3.

19. A solution according to claim 18, wherein the sight glass is a borosilicate glass.